cifs: Possible slab memory corruption while updating extended stats (repost)

Updating extended statistics here can cause slab memory corruption
if a callback function frees slab memory (mid_entry).

Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>